Add parsing tests to expression language.

**Is your feature request related to a problem? Please describe.**

There are no tests to make sure the grammar is correct.

**Describe the solution you'd like**

Add sufficient tests to validate the grammar. You can add the parsing tests to the `com.intuit.dsl.expression.tests` project

[Sample test](https://github.com/intuit/common-xtext-expression-language/blob/develop/com.intuit.dsl.expression.parent/com.intuit.dsl.expression.tests/src/com/intuit/dsl/tests/ExpressionParsingTest.xtend)

**Additional Context**
[Xtext Unit testing] (https://www.eclipse.org/Xtext/documentation/303_runtime_concepts.html#testing)

Start with com.intuit.dsl.expression.parent/com.intuit.dsl.expression.tests/src/com/intuit/dsl/tests/ExpressionParsingTest.xtend and the linked Xtext unit-testing guidance. Add parsing tests in the com.intuit.dsl.expression.tests project that cover the expression grammar; done means the grammar has sufficient validation tests and they pass.

Written by the indexing model from the issue text.